1. Field of the Invention
This invention relates to removing drilling fluid from well bores with aqueous removal solutions, and more particularly, to aqueous removal solution spacers utilized between drilling fluids and hydraulic cement compositions which remove drilling fluid from well bore surfaces and prevent cement composition contamination.
2. Description of the Prior Art
A variety of drilling fluids have been used heretofore in drilling subterranean well bores utilizing the rotary drilling method. The most commonly used such drilling fluids are water-based gels formed with clays and/or polymers. When a well bore is drilled using the rotary method, the drilling fluid is circulated downwardly through the drill string, through the drill bit and upwardly in the annulus between the walls of the well bore and the drill string. The drilling fluid functions to maintain hydrostatic pressure on formations penetrated by the well bore and thereby prevent blow-outs and, to remove cuttings from the well bore. As the drilling fluid is circulated, a filter cake of solids from the drilling fluid forms on the walls of the well bore. The filter cake build-up is a result of initial fluid loss into permeable formations and zones penetrated by the well bore. The filter cake and gelled or partially gelled drilling fluid mixed therewith reduce or prevent additional fluid loss as the well is drilled.
After the well bore reaches its total depth, the drilling and circulation of drilling fluid are stopped and a string of pipe, i.e., casing, is run into the well bore. After the pipe is run, the well bore is conditioned by circulating drilling fluid downwardly through the interior of the string of pipe and upwardly through the annulus between the pipe and the walls of the well bore. The purpose of the conditioning is to remove as much of the filter cake and gelled or partially gelled drilling fluid from the walls of the well bore as possible. However, at the end of the conditioning, drilling fluid remains on the surfaces of the well bore and on the string of pipe therein.
Primary cementing operations are next performed in the well bore, i.e., the string of pipe disposed in the well bore is cemented therein by placing a hydraulic cement composition in the annulus and allowing it to set into a hard impermeable mass. The hydraulic cement composition is placed in the annulus by pumping the cement composition through the interior of the string of pipe and in the opposite direction into the annulus. The hydraulic cement composition displaces the drilling fluid from the string of pipe and the annulus whereby the annulus is filled with the hydraulic cement composition which is allowed to set therein.
A problem which is encountered in the above described cementing procedure is that because drilling fluid remains on the surfaces of the well bore and on the surfaces of the string of pipe in the annulus, a strong bond between the hydraulic cement composition after setting and the surfaces in the well bore does not result. This, in turn, allows pressurized formation fluids to enter and migrate through the well bore as well as other undesirable consequences.
In order to solve the above described problem, aqueous spacer fluids containing surfactants and other chemicals have been included between the drilling fluid and hydraulic cement compositions. As the drilling fluid is displaced, the aqueous spacer fluid contacts the drilling fluid remaining on the well bore and pipe string surfaces to remove the drilling fluid therefrom. While such spacer fluids have been used successfully to remove a major portion of the drilling fluid from the surfaces, all of the drilling fluid has often not been removed and the surfaces have not been left water-wet. As a result, the strength of the bond between the surfaces and the set hydraulic cement composition in the annulus is often not as strong as it needs to be to prevent the entry of pressurized formation fluids into the well bore and the deterioration of the cement bond.
Thus, there is a need for improved water-based drilling fluid removal solutions for use in well bores which provide enhanced removal of drilling fluid therein and leave the surfaces in the well bores water-wet whereby a very strong bond between the surfaces and the hydraulic cement compositions placed therein is achieved.
By the present invention improved methods of removing water-based drilling fluids from surfaces in a well bore and leaving the surfaces water-wet are provided which meet the needs described above and overcome the deficiencies of the prior art. A method of the invention is basically comprised of the following steps. A water-based drilling fluid removal solution comprised of water and a mixture of a sulfonated kraft lignin salt and an N-methyl-N-oleyltaurine salt dissolved therein is provided. The water-based drilling fluid is displaced from the well bore and the well bore surfaces are contacted with the removable solution to thereby remove the drilling fluid from the surfaces and make the surfaces water-wet.
Another method of removing a water-based drilling fluid from a well bore having a pipe string disposed therein, making the surfaces of the pipe string and the well bore water-wet and placing a hydraulic cement composition in the annulus between the pipe string and the walls of the well bore is provided by the present invention. Upon setting, the hydraulic cement composition readily bonds to the surfaces of the pipe string and the well bore and seals the annulus. This method basically comprises the following steps. A water-based drilling fluid removing spacer is provided comprised of a water solution having a mixture of a sulfonated kraft lignin salt and an N-methyl-N-oleyltaurine salt dissolved therein. The spacer followed by a hydraulic cement composition is pumped through the pipe string and through the annulus between the pipe string and the walls of the well bore so that the cement composition is placed in the annulus. Simultaneously, the drilling fluid and the spacer are displaced through and removed from the annulus. As a result, drilling fluid is removed from the well bore and pipe string surfaces forming the annulus and the surfaces are made water-wet to the hydraulic cement composition. The hydraulic cement composition in the annulus is then allowed to set.
The objects, features and advantages of the present invention will be readily apparent to those skilled in the art upon a reading of the description of preferred embodiments which follows.